Table of Contents

Overview

New Hampshire’s Sea Level Is Rising

The sea level off New Hampshire’s coast is up to 7 inches higher than it was in 1950.1 This increase is mostly due to land sinkage2 Solutions can be complicated because a lot of important infrastructure on the coast is shared by both city and state, so mitigation efforts will need to be coordinated.3 There are already over 2,544 properties at risk from from tidal flooding in NewHampshire.4 The state is planning over $600,000 in sea level rise solutions, which include restoration projects, research and outreach, flood prevention for roads, and fortifying coastal wetlands.

Sea level rise is speeding up

The sea level around nearby Boston, Massachusetts, has risen by around 7 inches since 1950. Its speed of rise has accelerated over the last ten years and it’s now rising by about 1 inch every 10 years.1 Scientists know this because the sea level is measured every 6 minutes using equipment like satellites, floating buoys off the coast, and tidal gauges to accurately measure the local sea level as it accelerates and changes.5

Created with Highcharts 6.2.0

Sea level measurement from Boston area tide gauge since 1950

Causes & Complications

Why Are Sea Levels Rising?

While there are four main causes of sea level rise in New Hampshire, land sinkage is the largest contributor.1 Because the land is getting lower each year, due to a process called subsidence, New Hampshire is particularly vulnerable to an increased rate of sea level rise in the future. Click here to learn more about these causes.

How Land Sinkage Increases Sea Level Rise

Most flooding happens in the winter

The highest tides in New Hampshire occur during nor’easters. These wintertime storms push more water to the coast, raising high tides even higher, especially when combined with an increased gravitational pull from the moon. In places like Boston, these tides are typically over a foot and a half higher than normal high tides.1 Add that to the 7 inches of sea level rise since 1950 and you end up with flooding even on sunny days.

Solutions aren’t simple

Solving for sea level rise can be complicated in New Hampshire because most mitigation efforts will need to be organized as joint efforts. New Hampshire has a lot of municipality and state shared infrastructure, so mitigation efforts must be coordinated in order to succeed.3 In addition, the state’s coastal habitats and communities are both at risk from sea level rise and flooding, but solutions for each affect the other. The Hampton-Seabrook Estuary, for example, is host to 5,000 acres of tidal flats and coastal estuaries, which provides a critical habitat for important wildlife and serves as a buffer to protect inland communities.6 For the estuary to survive, protective barriers such as seawalls will need to be removed so waters can shift inland. However, there are existing communities in the areas the estuary would shift into, and these communities may block the removal of these barriers.7

Forecast

What's the Future of Sea Level Rise?

In the last decade, sea level rise around New Hampshire has accelerated, and is now rising by as much as 1 inch every 10 years. The longest-running tide gauge nearby is in Boston, MA where it took 36 years for the sea level to rise around 6 inches.1 Scientists now forecast that in just the next 15 years, the sea level will have risen by another 6 inches.8

Scientists are not certain how fast the ocean will warm and ice will melt. They expect water levels to continue to rise faster, but are not sure just how fast. Therefore scientists from the National Oceanic and Atmospheric Administration (NOAA) and the US Army Corps of Engineers (USACE) have made predictions based on ranges from low to high.

Below you can see the range of the NOAA and USACE high and intermediate forecasts for various locations around New Hampshire.9 Currently, the USACE high forecast, seen as the darkest red line, is the most likely projection.

Created with Highcharts 6.2.0National Sea Level Rise Forecasts

Flooding

Why Are Floods More Frequent?

When the ocean rises high enough, high tides cause flooding even on sunny days. Even though the sea level has only risen by around 2 and a half inches, tidal flooding has increased by 260% in some areas of New Hampshire since 2000.10

How sea level rise affects New Hampshire

Flooding even when there’s no rain

Drainage systems are designed to channel excess rainwater from the streets and drain it into the sea. But with the pressure from rising sea levels and higher tides, seawater can get pushed into these pipes and spill out into the streets. This causes flooding even on days without rain.

Drainage Under Normal Tidal Conditions

Drainage With High Tide / Sea Level Rise

Increased storm surge flooding

Unfortunately, slightly higher sea levels make hurricanes even more damaging. Just a few more inches of sea level rise allow a hurricane to push more water onto the land, even if the hurricane itself doesn’t make landfall.

Hurricane Sandy, 2012

Higher sea levels create a higher launching point for storm surge. These small changes in sea level rise are enough to turn what were 100-year storm surges into much more frequent events. In fact, in a third of 55 coastal sites studied throughout the US, 100-year storm surges will be 10-year or more frequent events by 2050.11

This means that in many coastal cities, if you bought a house with a 30-year mortgage today, by the time you paid off your mortgage you could be experiencing extreme 100-year storm surges ten times more frequently due to sea level rise alone. This does not include the added risk of more intense storms resulting from warmer water and a warmer atmosphere, which could further increase storm surge damage.

In 2012, Hurricane Sandy hit New Hampshire, bringing powerful storm surge that caused over $600,000 worth of damage to the state.12 Without sea level rise, Hurricane Sandy’s storm surge of around 6 feet would have been much lower.13

Hurricane Sandy, 2012

Costs

What's at Risk in New Hampshire?

There’s a lot at risk from sea level rise and flooding in New Hampshire. Even though the state has the least amount of tide-influenced coastline in the country, with less than 20 miles, the coast and its wetlands support diverse habitats that are at great risk from sea level rise.14 Birds such as the Osprey, Northern Harriers, Least Terns, and more depend on the Hampton-Seabrook Estuary, however rising seas have caused breeding grounds to disappear, triggering a rapid decline in numbers and putting the species at risk of extinction.6

In addition, there are many assets and infrastructure on the coast of New Hampshire that are shared by both the state and municipalities, including roads and wastewater treatment facilities. Failure to coordinate solutions for the effects of sea level rise could worsen flooding and storm surge disasters statewide.3

New Hampshire is spending over $600,000 on solutions

Some cities have sufficient resources to deal with this problem while others do not. Delaware will need solutions at the individual, local, state, and federal levels to protect its coastal communities.